1. Field of the Invention
The present invention relates to a toner image fixing method for fixing a thermofusible toner image formed by electrophotography, electrostatic recording, electrostatic printing, and the like, and more particularly to a toner image fixing method for fixing a thermofusible toner image which has a relatively low fixing temperature and high tackiness when melted, such that the image is easily and completely fixed without unwanted adhesion to a fixing member of a fixing unit of an image forming apparatus.
2. Discussion of the Background
In an image forming apparatus using electrophotography, an image is generally formed according to the following steps:
(1) an electrostatic latent image is formed on an electrophotoconductor by various methods;
(2) the electrostatic latent image is developed with dry toner;
(3) the developed toner image is transferred to an image supporting material such as paper, for example; and
(4) the transferred toner image is fixed by application of heat and pressure to obtain a fixed toner image.
A heat roller fixing method is widely used as a fixing method for fixing a developed image with dry toner (hereinafter referred to as toner) because of the high heating efficiency. In a conventional heat roller fixing unit, a rotating heat fixing roller contacts a developed toner image on an image supporting material and forms a fixed toner image upon application of pressure. Immediately after the toner image on the image supporting material is released from the rotating heat fixing roller, the toner image is melted and tacky. If the releasability of the surface of the heat fixing roller from the toner image is insufficient, so-called "hot offset" occurs in which the melted toner image is transferred to the heat fixing roller and subsequently retransferred to another region on the image supporting material or to a following image supporting material.
In attempting to solve this problem, materials such as fluorine-containing polymers and silicone polymers, which have good releasability because of their relatively low surface free energy, have been proposed for use as the surface material of the heat fixing roller. For example, heat fixing rollers having a fluorine-containing polymer surface have been disclosed for use in so-called "oil-less" fixing methods in which the heat fixing roller does not require oil as a releasing agent.
Specific examples include:
(1) a heat fixing roller in which a resin layer containing carbon fluoride in an amount of 1 to 25% is formed on a metal roller (Japanese Laid-Open Patent Application No. 57-54968); PA0 (2) a heat fixing roller in which a layer of a fluorine-containing rubber and resin, having a content of fluorine-containing resin of 5 to 70% by weight, is formed on a metal roller (Japanese Laid-Open Patent Application No. 58-2864); PA0 (3) a heat fixing roller in which a perfluoroalkyl silicone compound having a CF3 group at an end of the polymer molecule is formed on a metal roller (Japanese Laid-Open Patent Application No. 61-36778); PA0 (4) a heat fixing roller having thereon a complex plating layer containing fine particles having lubricating and releasing properties (Japanese Laid-Open Patent Application No. 61-205966); PA0 (5) a heat fixing roller in which a fluorine-containing silicone compound is layered onto a roller substrate (Japanese Laid-Open Patent Application No. 4-273276); and PA0 (6) a heat fixing roller in which tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA) resin having a CF3 group at an end of the polymer molecule is layered onto a roller substrate (Japanese Laid-Open Patent Application No. 5-72934). PA0 (1) hot-offset often occurs at relatively low fixing temperatures of less than 150.degree. C., depending on the surface condition of the heat fixing rollers; and PA0 (2) a fixable temperature range, which represents the difference between a minimum toner fixing temperature and a minimum temperature below which hot-offset occurs, is often too narrow to be controlled by a conventional fixing unit. Therefore these heat fixing rollers are not practical to use. PA0 (1) a heat fixing roller whose surface has a center-line average roughness of greater than 0.5 .mu.m (Japanese Laid-Open Patent Application No. 62-168182); and PA0 (2) a heat fixing roller whose surface has a ten-point mean roughness of 1 to 100 .mu.m and a mean peak-to-peak length of 1 to 100 .mu.m (Japanese Laid-Open Patent Application No. 64-29882). PA0 (1) a heat fixing method in which fixed toner is released from a fixing unit after cooling (Japanese Laid-Open Patent Application No. 5-341672); and PA0 (2) a heat fixing method that uses a heat fixing roller and a pressure roller pressing the heat fixing roller, in which the heat fixing roller is rotated when the temperature of the heat fixing roller is lower than a specified temperature to avoid an uneven temperature distribution in the heat fixing roller (Japanese Laid-Open Patent Application No. 7-92851).
However, these heat fixing rollers have, for example, the following disadvantages:
In addition, methods for preventing hot-offset have been proposed, in which a physical property of the surface of a heat fixing roller is specified to obtain good releasability. The methods include:
However, these specified physical properties are measured in the absence of heat and pressure, so that these specified physical properties are not correlative with the adhesive and release properties of toner to a surface of a fixing roller when heat and pressure are applied thereto. Therefore, hot-offset is not solved by using these fixing rollers.
Further, a material having a relatively low static contact angle has been proposed for use in a heat fixing roller. However, the dynamic aspects of toner transfer in a heat fixing process is not correlative to merely a single factor of the static contact angle.
Furthermore, various constructionally improved fixing units have also been proposed, such as:
These constructional improvements to the fixing unit require modification of the entire image forming apparatus, which results in an increase in the manufacturing cost of the image forming apparatus.
Recently, the heat energy used for fixing has decreased because of the desire for high speed fixing for high speed recording and also to save heat energy. Accordingly, toner is required to have good adhesion to a recording material even at a relatively low fixing temperature. Therefore, the toner has to include a tacky resin having a relatively low softening point in order to have good thermosensitivity. However, this also results in the toner adhering to the surface of a heat fixing roller, which would normally have good releasability when used with a conventional toner having a normal fixing temperature, thus resulting in the occurrence of hot-offset.
Because of these reasons, a need exists for a toner fixing method that produces a good fixed image without hot-offset even when toner having a relatively low fixing temperature is used.